The role of abnormal red cell aggregation in the development of diabetic retinopathy is being investigated through a study of the clinical picture coordinated with measurement of plasma components and rheological parameters. The Wells-Brookfield rheoscope is used to classify the degree of red cell aggregation, and sedimentation rates are measured for 40% suspensions at 37 degree C. Viscosity of serum, plasma, and 40% red cell suspension are measured in a Wells-Brookfield microviscometer. Rigidity of red cell aggregates is measured using a centrifuge to determine a modulus of rigidity, and resistance to flow is measured by forcing 40% red cell suspensions to flow through a series of filters at a constant flow rate. Statistical results to date indicate that levels of fibrinogen and alpha-2 globulin rise while albumin level falls as one moves from the control group into groups of diabetics with increasingly severe retinopathy. There is a corresponding rise in red cell aggregation and sedimentation rate, and a small increase in plasma viscosity, as well as increasing retinal ischemia. So far about 165 subjects have been tested. Correlations and statistical significance appear to be more pronounced for the females.